In the field of soldering pipe ends together, one end of a pipe is inserted into the open end of another, larger pipe--in practice the "larger" pipe end is usually a "fitting" connected at its other end to another pipe end. After the pipe end is positioned within the larger pipe end, i.e. the fitting, the overlapped ends are heated and a soldering material is applied to the exposed end of the overlapped portion. Because of capillary action and the heat, the soldering material flows axially between the pipe ends from the first axial end of the overlap all the way to the other end of the overlap. This complete flow from one end of the overlapped portion to the other end thereof is generally referred to as 100% flow, and is generally the most desirable condition since it provides a joint of maximum strength.
However, under certain conditions, such 100% flow is undesirable. As one example, in certain manufacturing processes such as the manufacture of electronic components, it is necessary to supply a gas such as a nitrogen gas in a form as pure as possible. In such an environment, if the solder composition (or the flux material which is applied to the joint in advance of the soldering material and generally moves through the space ahead of the soldering material) reaches the end of the overlapped portion and protrudes into the interior of the conduit, it provides a source of impurities which can be carried away with the gas and adversely affect that manufacturing process. Under these conditions, it is desirable if not mandatory to limit the solder flow to less than 100%, for example between 25% and 75% for example, 50%.
The desire to limit solder flow to less than 100% of the overlapped portion has been recognized on occasion heretofore. For example, the Lindquist U.S. Pat. No. 1,890,998 recognizes the problem and provides one attempted solution therefor. However, the problem is a most difficult one to solve and to applicant's knowledge has not been solved heretofore as satisfactorily as with the present invention.
The difficulty is that once the pipe ends are heated to the proper temperature, which for a silver soldering composition would be approximately 1100.degree. F., the force of the capillary action carrying the soldering material through the space between the pipes is a very great force and difficult to stop. Attempts to limit this flow by controlling and varying the temperature along the overlapped portion, i.e. 1100.degree. F. for a first portion and a cooler temperature for the latter portion of the overlap, have also met with failure because of the extreme difficulty in attempting to provide an abrupt change in temperature in a heat conductive material such as the metal of the pipes. Indeed, in process piping the material is often copper which is of course is extremely conductive, thereby rendering solder control by temperature control very difficult if not impossible.
The applicant also attempted to limit solder flow by mechanically forcing the two pipes together along a circumferential band at the axial location where it was desired to stop the solder flow. (This attempt is essentially analogous to the attempt in the Lindquist U.S. Pat. No. 1,890,998 of utilizing a tapered end on the outer pipe.) However, this attempt also failed. No matter how great the force urging the inner and outer pipes together, there will always remain a "space" of a few thousands of an inch through which silver solder will flow under the high temperature conditions mentioned above. If on the other hand the force urging the two pipes together is extremely large, it will in all likelihood distort one of the pipes relative to the other, thereby creating at some point around the circumference of the overlapped pipes an opening through which the solder can flow to the end of the overlap and into the conduit.
Hence, there exists a need for a method of soldering pipe ends together, and to such a joint, wherein the flow of solder can reliably be stopped at a predetermined axial location less than the full axial length of the overlap.